Many manufacturing processes employed in producing plastics, textiles, paper and composites of them, as well as other generally non-metallic goods, involve forming or processing the product from continuous web or webs. The individual webs are wound on reels, and as each reel is processed in the manufacturing machinery, the web on the next reel must be spliced to it, in order to maintain web continuity, and avoid shutting down and restarting the machinery.
The end product must often be screened to reject any of it containing splicing material. In some cases this screening or inspection can be performed optically, e.g. by photosensors which detect specific properties such as color, reflectivity, or the like, designed into the splicing tape for that purpose.
A splicing tape for industrial application must have an adhesive with good bonding properties. The tape itself must be thin, strong and preferably elastic. The requirement of thinness is dictated by the clearance between rollers in the manufacturing machinery. In order not to damage the machine, the tape thickness must be much smaller than the web thickness, so that the splice can go through the machinery essentially the same way as the unspliced web.
A popular splicing tape consists of a 1 mil polyester film coated with approximately 1 mil of adhesive. A vacuum evaporated layer of aluminum approximately 100 microinches thick is often added for splice detection by optical reflectivity. Such a tape will be referred to as aluminized splicing tape.
An example of a composite process using aluminized splicing tape for a continuous feed of web material is the manufacture of pre-packaged adhesive bandages. Primary raw materials come in a web which may be as wide as six feet. In primary processing the wide web is slit into narrow. e.g., 1 inch, strips which are wound on individual reels. In the slitting operation several wide reels are spliced together to get the required length of the narrow strip.
The various materials in form of web strips are then fed continuously into bandage manufacturing machinery by splicing the end of one roll to the beginning of the next. The machinery can perform bonding, perforation, folding, and interleaving of the individual web strips until, in the last stage, the composite is cut into individual bandages. It is clear that the end product can contain splices originating from the primary slitting operation or from subsequent joining of the narrow web strips.
In the bandage manufacture, and in many other instances, the product is such that the splices on it may be covered up and hence are optically inaccessible for detection (referred to herein as an interior splice). Prior to the present invention, no known inspection technique was capable of satisfactorily detecting aluminized splicing tape in the interior of a product, i.e. when it was not optically accessible. In limited instances, metal detectors have been employed to detect interior aluminized splices in the end product. However, ordinary metal detectors, which operate in the megahertz region, have not generally met with industrial acceptance because of sensitivity to extraneous conditions such as moisture. The reliability of internal splice detection by ordinary metal detectors could be improved if much thicker metal coating, or a metal tape, could be used for splicing so that the metal thickness would be comparable with the skin depth of the current generated by such detectors. However, as pointed out earlier, increased thickness of splicing tape is undesirable and often unacceptable.